This application relies for priority upon Korean Patent Application No. 2001-15150, filed on Mar. 23, 2001, the contents of which are herein incorporated by reference in their entirety.
The present invention generally relates to a method of making a semiconductor device and, more specifically, to a method of forming a metal gate electrode.
A gate electrode is fabricated by forming a conductive film having a uniform thickness on a semiconductor substrate and subsequently etching the conductive film to be a predetermined shape. Generally, polysilicon has been used to make a typical gate electrode due to its excellent interface characteristics with a gate oxide at a high temperature. However, as the semiconductor devices are becoming highly integrated, the typical polysilicon gate electrode cannot satisfy manufacturers of the semiconductor devices with its operation speed and sheet resistance. Thus, a metal gate electrode comprised of a polysilicon and a refractory metal such as tungsten formed on the polysilicon has been used. However, the metal gate electrode has a problem due to an abnormal oxidation that is caused by excellent oxidation characteristics of tungsten used for metal gate electrodes.
FIGS. 1A, 1B, 2A and 2B are cross-sectional views showing a method of making a metal gate electrode in accordance with the conventional art.
First, as shown in FIG. 1A, a gate oxide layer 12 is formed on a semiconductor substrate 10. Then, a polysilicon layer 14, a tungsten layer 16 and a gate electrode capping layer 18 are sequentially formed on the gate oxide layer 12. Further, a conductive barrier layer (not shown) is formed between the polysilicon layer 14 and the tungsten layer 16 to prevent a chemical reaction therebetween. Next, a gate electrode 20 is formed by sequentially etching the gate electrode capping layer 18, the tungsten layer 16, the polysilicon layer 14 and the gate oxide layer 12. Subsequently, an oxidation process is performed to cure damage 22 on the semiconductor substrate 10 and ensure reliability of the gate oxide layer 12. As a result, as shown in FIG. 1B, the damage 22 is cured and an oxide layer 12a is formed on the semiconductor substrate 10 and side walls of the polysilicon layer 14 and the gate oxide layer 12. However, an abnormal oxidation 12b is caused on side walls of the tungsten layer 16 because an oxidation rate of the tungsten layer 16 is higher than that of the polysilicon layer 14. Accordingly, to prevent the abnormal oxidation 12b during forming a metal gate electrode, a selective oxidation process has been developed. The selective oxidation process oxidizes only silicon layer but does not oxidize a metal layer after patterning a metal gate electrode. The selective oxidation process is performed by adjusting partial pressures of hydrogen gas and H2O to selectively oxidize the silicon layer. Further, partial pressures are adjusted by controlling oxygen gas and hydrogen gas of a source gas. For example, the selective oxidation process such as wet hydrogen oxidation selectively oxidizes silicon by controlling the following chemical reaction.
Si+2H2O⇄SiO2+2H2xe2x80x83xe2x80x83(1)
W+3H2O⇄WO3+3H2xe2x80x83xe2x80x83(2)
That is, by controlling the partial pressures of H2O and the hydrogen gas, reactions are favored toward the right side in equation 1 and in the direction of the left side in equation 2, respectively. Therefore, the oxidation of tungsten can be prevented.
However, the selective oxidation process described above has a narrow process margin, and it is difficult to adjust the partial pressures of the hydrogen gas and H2O to only oxidize the silicon. Accordingly, as shown in FIG. 2A, a small amount of the tungsten is oxidized, so that tungsten oxide 12c is formed on side walls of the tungsten layer 16. As shown in FIG. 2B, the insulative tungsten oxides 12c causes whiskers 24 due to a thermal energy that is applied to the tungsten oxide 12c during a heating process of subsequent semiconductor device manufacturing processes. Thus, an electrical short between the gate electrodes adjacent each other can be caused by the whiskers. The whiskers 24 are formed due to an amorphous phase and nucleation cites on the surface of the tungsten oxide 12c. That is, surface mobility of the tungsten oxide 12c having amorphous phase is increased by the thermal energy during a beating process, then the amorphous tungsten oxide 12c is moved toward the nucleation cites and crystallized at the nucleation cites, so that the whiskers 24 are formed. Accordingly, it is required to completely suppress the oxidation reaction of the tungsten during the selective oxidation process.
It is, therefore, an object of the present invention to provide a method of forming a metal gate electrode that is capable of preventing whiskers by ensuring reliability of a selective oxidation process.
In accordance with one aspect of the present invention, there is provided a method of making a metal gate electrode wherein a selective oxidation process to be performed after patterning the metal gate electrode is carried out under a nitrogen containing gas ambient to ensure a reliability of the selective oxidation process.
In the method of making the metal gate electrode according to the present invention, the nitrogen containing gas prevents a metal layer of the metal gate electrode from being oxidized. The reason is that nitrogen is unstably combined with the metal layer having high reactivity, so that it prevents the metal layer from reacting with oxygen. Further, the nitrogen containing gas suppresses formation of the nucleation cites on a metal oxide layer and decreases surface mobility of the metal oxide layer, so that the whiskers are not formed during a subsequent heating process.
In accordance with the method of the invention, a metal gate electrode having a silicon layer, a conductive barrier layer and a metal layer is formed. A metal gate electrode pattern including the silicon layer, the conductive barrier layer and the metal layer is formed. The selective oxidation process is performed on the metal gate electrode pattern in the nitrogen containing gas ambient.
In the method of making the metal gate electrode, one of nitrogen N2, nitrogen monoxide NO, nitrogen oxide N2O and ammonia NH3 is used as the nitrogen containing gas. The gases are used by themselves, or one or more of the gases are mixed with each other.
In the method of making the metal gate electrode, the selective oxidation process can be performed at a temperature lower than that of a conventional thermal oxidation process, so that the process margin is increased.
The present invention will be better understood from the following detailed description of the exemplary embodiment thereof taken in conjunction with the accompanying drawings, and its scope will be pointed out in the appended claims.